These studies utilize 3H-thymidine autoradiography, neuroanatomical tract tracing and cell counting techniques to study neuronogenesis in the adult dentate gyrus. We have found that the dentate granule cells which are generated in adult rats can extend axons through an apparently mature neuropil, and for appreciable distances. In order to determine whether new neurons are added to the adult primate brain, we are studying neuronogenesis in the dentate gyrus of pygmy marmosets.